In UMTS LTE the downlink control channel PDCCH (Physical Downlink Control Channel) carries information such as resource allocation for uplink or downlink transmission. A PDCCH message can use 1, 2, 4 or 8 Channel Control Elements (CCEs or resource elements)—referred to as CCE aggregation levels 1, 2, 4 or 8. PDCCH messages can be transmitted using one of a set of available message formats (e.g. with different signal characteristics such as number of information bits and channel coding rate, The different formats are referered to in the LTE specifications as “DCI formats”. In addition, different destinations or purposes for the PDCCH messages may be indicated by different scrambling sequences applied to the message CRC (in the LTE specifications the different scrambling sequences correspond to different identities, referred to as RNTIs). Different RNTIs are used to distinguish UE-specific PDCCH messages intended for a particular UE, from Common PDCCH messages intended for reception by more than one UE. In the case of a PDCCH message intended for reception by more than one UE, this could be for a defined group of UEs, or any UE.
A mobile station, like a UE in LTE, does not know in advance the location in CCE space of messages intended for it. In principle, the mobile station could attempt to blindly decode all the possible PDCCHs with different starting positions in the CCE space and thus receive any messages intended for that mobile station. However, if the CCE space is large the processing complexity is prohibitive. Therefore a more limited search is configured which consists of a number of search spaces. A UE may search a given search space for messages with one of more DCI formats. For simplicity of explanation, we may consider only one DCI format and one RNTI in a UE-specific search space, but the same discussion is applicable for multiple DCI formats, multiple RNTIs and for UE-specific and Common search spaces.
A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads (DCI formats and RNTIs) that it assumes may be transmitted for that aggregation level. The set of PDCCH payloads that the UE assumes may be transmitted may be a subset of all possible PDCCH payloads defined by the LTE specification. Search spaces are defined per aggregation level; a secondary station thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3,4,5,6,7,8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1,2, . . . 8 and 9,10, . . . ,16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs.
The LTE specification currently requires the UE to perform the following in a search spaces designed for UE-specific PDCCH messages on a single carrier:                6 decoding attempts of 1-CCE aggregation        6 decoding attempts of 2-CCE aggregation        2 decoding attempts of 4-CCE aggregation        2 decoding attempts of 8-CCE aggregationIn addition the UE is required to perform the following in a search spaces designed for Common PDCCH messages on a single carrier:        4 decoding attempts of 4-CCE aggregation        2 decoding attempts of 8-CCE aggregationIn general, the UE may be required to search one or more sets of search spaces which could be considered as being grouped together to form one or more search space structures.        
The larger aggregations are intended to be used for large messages, and/or small messages when a lower code rate is required, for example under bad channel conditions. However, restricting the search spaces to reduce processing complexity limits the availability of suitable aggregations for different radio channel conditions as those conditions vary.
In the case that the system supports the use of multiple carrier frequencies simultaneously, then one possible method of operating the above described system would be to send a PDCCH on each carrier that is to be used for data transmission. In this scenario, it would be desirable to limit the processing power required to search for the possible PDCCHs.